A normal sinus cardiac rhythm arises from the sinoatrial (SA) node which acts as the heart's natural “pacemaker”. Cardiac rhythms can arise from the atrioventricular (AV) node during periods of significant sinus bradycardia or complete AV block. The intrinsic rate originating from the AV node overtakes the intrinsic rate originating from the SA node. Rhythms originating from the AV node are commonly referred to as “junctional rhythms”, “nodal rhythms” or “junctional ectopic tachycardia” (JET). Junctional rhythms can be highly symptomatic due to loss of AV synchrony. Symptoms may include shortness of breath, choking sensation, chest pain, fatigue, anxiety, dizziness, and confusion, all of which are generally considered to be signs of decreased cardiac output.
If junctional rhythms produce atrial beats closely followed by ventricular beats, dual chamber pacemakers may sense the atrial beat but the closely following sensed ventricular beat causes the pacemaker to presume the rhythm is normal sinus rhythm, and no pacing will occur. Similarly, junctional rhythms in which the ventricular beat is sensed before the atrial beat also result in no pacing as the pacemaker is inhibited by the series of ventricular sensed events. The junctional rhythm, which may be sporadic, remains undiagnosed with accompanying symptoms unexplained. Junctional rhythms remain challenging to manage using modern pacemakers. Ambulatory monitoring of patients using rhythm monitors designed to detect junctional rhythms has demonstrated that junction beats and junctional rhythms exist with a far higher frequency than previously understood in typical pacemaker or defibrillator (ICD) patient populations. A need remains, therefore, for an implantable cardiac pacing device and associated method for treating patients that experience junctional rhythms.